SPID#: 24 The aim of this study was to test the hypothesis that CTLA4-lg will prolong survival of renal allografts in a nonhuman primate model. CTLA4- lg has been shown to modify the allograft rejection response to produce prolonged graft survival and/or induce specific unresponsiveness in rodent models. Adult rhesus macaques (11.8 - 13.5 kg) were utilized for these studies. Ten animals received an allogenic renal transplant, a left nephrectomy and simultaneous right contralateral ureter ligation. The paired transplants were performed simultaneously by exchange of kidneys between recipient individuals. One recipient in each pair was treated with CTLA4-lg and the other served as a control and was treated with human albumin. Renal allografts in the control treatment group promptly failed (4-8 days) and histologic analysis suggested both a cellular and humoral immune mediated mechanism. CTLA4-lg treatment in one of four recipients was associated with marked prolongation of allograft survival. One CTLA4-lg treated recipient died secondary to necrosis of the distal transplant ureter and was not included in the analysis. In summary, the results of this pilot study suggest that CTLA-4lg has the ability to prolong primate renal allograft survival. Further studies are required to determine the significance of this initial observation.